With between 1.5 and 5 million people reportedly expected to witness Barack Obama’s inauguration firsthand on Tuesday, the sheer volume of visitors in Washington D.C. could well rival Mecca during the Hajj, the yearly Muslim pilgrimage.

Which is why various agencies involved in planning for the big event turned to Hani Mahmassani, a professor of civil and environmental engineering and the director of the Transportation Center at Northwestern, for advice about anything from facilitating access to preventing disasters.

Mahmassani said that the inauguration and the highly choreographed pilgrimage would have little in common except for the magnitude of their crowds.

“The density that you get in the Hajj are simply unheard of, so we’re not going to nearly approach anything like that,” he said.

Mahmassani has worked on the physics of crowd management since the 1980s, when he first studied crowds at the Hajj using mathematical models with one of his Ph.D. students at the University of Texas.

“It’s a fairly recent field, I would say,” Mahmassani said. “The scientific community has evolved in dealing with crowds, and it’s people from a variety of backgrounds, not all engineers. Some are physicists, some are psychologists, anthropologists and so on.”

For Mahmassani, one of the biggest challenges for the Inauguration will be the coordination between the agencies who have jurisdiction over the event.

“Different agencies have different responsibilities for different things, from homeland security to policing the traffic,” he said. “The truth is, they really are quite adapted to doing all these things … They are the real experts. My role is more to alert them of issues related to crowd dynamics and make sure that all angles are covered.”

Mahmassani also had advice for those who decided to spend the day on the National Mall.

“Be prepared to walk, be prepared to stand, be patient. Just enjoy the moment,” he said. “Try to not lock yourself inside a sea of people … If for some reason you have to get out, if you’re not feeling well, if you’re feeling dizzy or whatever, just have a way out.”